1. Field of the Invention
The present invention relates to a charged particle beam apparatus which employs a scanning electron microscope (SEM) to inspect and/or review defects on a sample surface. More particularly, it relates to a low-voltage scanning electron microscope (LVSEM) for inspecting and/or reviewing defects on surfaces of wafers or masks in semiconductor manufacturing industry.
2. Description of the Prior Art
In semiconductor manufacturing industry, defects can occur on surfaces of masks and wafers during semiconductor fabrication process. These defects impact yield to a great degree. Apparatuses or tools, which can do defect inspection and/or defect review that are typically based on microscopy, have been employed to monitor semiconductor manufacturing yield. The apparatuses are always desired to have high spatial resolution and high throughput. Since critical feature dimensions of patterns on wafer and mask shrunk to or even beyond the competent ability limit of photon microscopy, SEM has been widely employed. Comparing a photon microscope, SEM not only can detect smaller defect sizes due to its higher imaging resolution, but also is able to detect electric defects due to its charging ability.
FIG. 1A and FIG. 1B show a conventional SEM for defect inspection and/or review in semiconductor manufacturing. In FIG. 1A, a primary electron (PE) beam 2 is emitted from an electron source 1 and accelerated to a higher energy by an anode 3. A fixed gun aperture 4 limits the current of the beam 2 to a desired value. Next, the PE beam 2 is focused to become a small probe spot on the surface of a sample 12 by a condenser lens 5 and an objective lens 10. The focusing power of the condenser lens 5 and the opening size of a final beam-limit aperture 6 are selected to get a desired probe current and make the probe spot size as smaller as possible. To obtain small spot sizes over a large range of probe current, usually the beam-limit aperture 6 has multiple openings with various sizes. As shown in FIG. 1B, secondary electrons (SE) and backscattered electrons (BSE) will emerge from the sample surface where the probe spot is, which form SE beam 13 and BSE beam 14 respectively. An in-lens detector 7 can collect SE and/or BSE. To collect BSE only, the voltage difference between the electrode 11 and the sample 12 are adjusted to reflect SE back to the sample surface. As the PE beam 2 is dynamically deflected by either or both of deflectors 8 and 9, the probe spot scans the sample surface and the SE and/or BSE images of sample surface can be obtained. A SE image and a BSE image comprise topographic information and material information respectively.
An interaction between each of primary electrons and a sample may generate radiation damage to the sample surface and makes exit areas of SE and BSE larger than the probe spot. To weaken the interaction, the primary electrons usually have energies at least lower than 5 keV and a SEM in such a case is especially named as LVSEM. However Coulomb interactions among primary electrons get stronger with decrease in electron kinetic energy, and the effect (Coulomb Effect) due to the Coulomb interaction will enlarge the probe spot size. Therefore in a LVSEM, primary electrons usually travel with high kinetic energies through most portions of their paths and are subsequently decelerated to desired low final landing energies just prior to impinging onto the sample, which is usually called as retarding technology.
Application requirements of defect inspection and/or defect review are diversified and changed with respect to new technologies emerging in semiconductor manufacturing. For example, to increase integration degree of IC chips, instead of further reducing feature dimensions on a wafer, 3-D chip integration is proposed and coming into market. 3-D integration can form highly integrated systems by vertically stacking and connecting various materials, technologies and functional components together. Various technologies for realizing 3-D integration are pursued. In 3D packaging technology, Die-to-die 3D integration is enabled by thinned die-to-die bonding and through-silicon-via (TSV) interconnections, and consequently requires defect inspection over a full TSV wafer. Therefore, TSV defect inspection especially requires a throughput much higher than the prior art. Accordingly, LVSEM for TSV defect inspection needs to have a probe current (such as several to tens uA) and a field of view (FOV) (such as several mm) both much larger than those of the prior art, such as ten or more times. Increasing probe current and FOV will dramatically increase the probe spot sizes all over the FOV in LVSEM. Although a TSV is typically several microns in diameter, which is much larger than feature dimensions on each die, reducing the probe spot size to match the TSV dimension is still difficult. Further, for inspecting patterns with large critical feature dimensions on a sample surface, an electron-beam (e-beam) inspection tool cannot compete with an optical inspection tool in throughput although it can detect electric defects but the optical inspection tool cannot.
As the probe current and the FOV increase, some minor issues in prior art of LVSEM become critical and must be solved, such as the Coulomb Effect due to the secondary electrons (SEs) and the backscattered electrons (BSEs) and the grey level uniformity of each of the SE and BSE images over the entire FOV. A super large probe current will incur a supper strong Coulomb Interactions among all the electrons on the path of primary electron beam from the electron source to the sample surface. In the prior art, the Coulomb Effect due to the SEs and BSEs can be ignored because the SE beam and the BSE beam are small in beam current (nearly equal to probe current) and large in beam size (several millimeters, in comparison with sub-millimeter size of the PE beam). Therefore, a lot of endeavors in designing LVSEM are focused onto eliminating crossovers of primary electron beam and less attention is paid to avoid the appearance of SE and BSE crossovers such as 15 and 16 in FIG. 1B.
The grey level uniformity of each of the SE and BSE images over a FOV mainly depends on the uniformities of the probe spot size of the PE beam and the detection ratio of the corresponding electrons. Over a FOV, the probe spot size will increase as the primary electron beam lands far from the FOV center due to the appearance and increase of off-axis aberrations. Off-axis aberrations of the primary electron beam increase with the off-axis shift R and the convergent angle α thereof on the sample surface. For a super large FOV, the field curvature (FC) aberration will dominate off-axis aberrations because it depends on R2·α as well as FC coefficient. In the prior art, the objective lens is put close to the sample as much as possible so as to reduce spherical and chromatic aberration coefficients. The working distance is usually not large than 2 mm In such a case, the angular magnification and the FC coefficient are very large, and both mean a large FC aberration.
SE and/or BSE beams scan the in-lens detector surface as the primary electron beam scans the sample surface. The scanning range on the detector depends on the FOV size, the imaging magnification of the objective lens and the deflection sensitivity of scanning deflectors both with respect to SE and BSE. Because the detection area of a detector cannot be too large due to limitation of electric respond characteristics, a super large FOV will require a relatively small magnification of the objective lens with respect to SE and BSE. As mentioned above, in the prior art, the objective lens is put very close to the sample to reduce spherical and chromatic aberration coefficients. In such a case, imaging magnification of the objective lens with respect to SE and BSE is very large, thereby limiting the available size of FOV due to the limitation on the detector size.
Accordingly, a new LVSEM, which can provide a probe current and a field of view (FOV) both much larger than those of the prior art, is needed. In addition, it is also desired to have SE image and BSE image simultaneously.